Cortical bone is a brittle, circumferentially laminated, multiphase composite structure that has been shown to exhibit microdamage similar to that found in advanced composite materials. Microcracks are an important manifestation of fatigue damage and are clinically associated with stress fractures, bone fragility, and remodeling. The goal of this study is to quantitatively investigate properties, such as fracture toughness and shear strength, of major interfaces in cortical bone, such as cement lines and interlamellar interfaces of osteons. Additionally, the effects of collagen fiber orientation and physiologic region on these fracture properties will be evaluated. The relevance to structural fracture resistance is that these bone interfaces have been shown to deflect microcracks, and may contribute to the overall fracture resistance of whole bones. The experiments are based on a standard fiber push-out procedure used for testing interfacial properties of fiber-reinforced composite materials. Human bone will be used to evaluate the dependence of interfacial properties of osteons as a function of collagen fiber orientation within the osteon and the adjacent matrix, and to assess the effect of the anatomic region. Equine bone will be used to evaluate differences between primary and secondary osteons. Micromechanics and constitutive modeling will be performed.